Rechargeable batteries are generally known and used in a variety of commercial, automotive, industrial and consumer applications where the use of compact, light weight, high capacity and extended charge life portable power sources are desirable. For certain applications, such as computers, electronic devices, and electric vehicles, both size and weight are critical factors in selection of a suitable battery material.
Current battery technology comprises essentially two general classes of batteries, liquid electrolyte batteries and solid electrolyte batteries. Polymer electrolyte batteries are generally considered as hybrid class of liquid electrolyte batteries. Liquid electrolyte battery technology is well known in the art. Typical commercial examples of these battery types are lead-acid, nickel cadmium, and nickel metal hydride cells and commercial lithium batteries.
In liquid electrolyte batteries, the electrolyte provides for ion transport between the cathode and anode. Typically, the amount of energy stored and retrievable from a conventional electrolyte battery is directly proportional to battery size and weight. For example, a Pb-acid automotive battery is capable of producing large amounts of current but such batteries typically have relatively low energy density and specific energy due their large volume and weight. Additionally, the corrosive liquid electrolytes employed by these batteries require complex packaging and sealing which add dead weight and dead volume. Since liquid electrolytes are employed in these batteries, their operating temperatures are generally limited by the freezing point and boiling point of the liquid electrolyte and they are unsuitable for applications in severe environments such as desert or artic climates, deep sea, high altitude or space applications.
More recently, advances in anode, cathode, and electrolyte materials and materials fabrication methods have led to the development of polymer electrolyte batteries and solid-state electrolyte batteries. While polymer electrolyte batteries offer improvements over conventional liquid electrolyte batteries due to weight and size reductions which result in reduction of dead weight and volume, these batteries generally exhibit similar corrosion problems as liquid electrolyte batteries where the corrosive electrolytes which are employed react with anodes and cathodes and lead to rapid degradation of battery charging performance, reversible charge capacity and charge cycle lifetime.
Solid state batteries have a number of preferred advantages over liquid electrolyte batteries and polymer electrolyte batteries. Since no corrosive electrolyte materials are employed, corrosion problems are eliminated and simplified packaging and sealing of battery cells is possible, eliminating unnecessary dead weight and volume. Due to the elimination of corrosion problems by employing solid-state electrolytes, electrolyte reactions with anodes and cathodes are eliminated resulting in stable charge capacities, high reversible charge capacity after extended cycling, and long battery lifetimes. Thus, solid-state batteries are theoretically capable of much higher energy densities and specific energies than liquid or polymer electrolyte batteries. In addition, solid-state batteries are capable of operating in temperature ranges which extend beyond either the freezing point or boiling point of a liquid electrolyte. For this reason, solid-state electrolyte batteries are particularly useful in severe environment applications in space, high altitudes, deep sea, desert or arctic climates.
Unlike commercial bulk batteries, which have relatively forgiving tolerances, the relatively slow solid-state ion diffusion kinetics and transport dimension constraints placed on electrolyte, anode and cathode film thickness and spacing in thin film, solid-state batteries impose demanding tolerances in the quality, structure, orientation and properties of as-deposited thin film electrolyte, anode and cathode layers. Since solid-state ion diffusion and transport through solid electrolytes is typically orders of magnitude slower than diffusion in liquid electrolytes, minimizing the thickness of the thin film electrolyte and the resultant spacing between anode and cathode is necessary for acceptable solid-state battery performance. Typically, the thickness of thin film electrolytes and spacing between electrodes in these batteries range from one to two microns in order to minimize ion diffusion distances and provide adequate transport kinetics for acceptable current densities. In contrast, typical electrolyte, anode and cathode dimensions and electrode spacing in commercial liquid and polymer electrolyte batteries generally range from hundreds of microns to tens of centimeters.
Since many candidate electrode materials for thin film batteries have hexagonal lattice structures, they are highly anisotropic and solid-state ion transport and diffusion kinetics are strongly dependent on crystallographic orientation. Thus, the crystallographic orientation of as-deposited films relative to the electrode-electrolyte interface is critical to efficient ion transport and optimum performance. For anisotropic hexagonal lattices, the fastest ion diffusion path is typically within the c-plane which is perpendicular to the c-axis. Ion diffusion parallel to the c-axis is generally orders of magnitude slower. This creates an imposing technical challenge in fabricating thin film electrodes as the orientation of as-deposited films is critical to acceptable performance. Since lattice anisotropy typically controls crystal nucleation and growth kinetics during film deposition, thin films deposited by conventional deposition methods typically retain preferential crystallographic orientation. Thus, for hexagonal materials deposited by conventional deposition processes, nucleation and growth typically occur with the c-axis perpendicular and c-plane parallel to the deposit substrate, an orientation which is particularly unsuitable for acceptable ion transport and battery performance.
In addition to the crystallographic orientation of electrode films, the interfacial contact area, orientation, and structure of electrode-electrolyte interfaces are critical for promoting rapid ion exchange between electrodes and electrolyte, eliminating ion buildup and transport bottlenecks within the cell, and minimizing cell impedance. Thus, deposition methods which maximize film layer interfacial contact area, provide for preferred or random film layer orientations, and produce intimate contact and bonding of layers are particularly desirable. By providing unimpeded ion transport across electrolyte-electrode interfaces, ion buildup and increased local potential created by high space-charge density, which may lead to decomposition of both electrolytes and electrodes, are eliminated. Such interfaces provide for thin film electrochemical cells and batteries having a high decomposition potential due to the enhanced stability of the electrolyte-electrode interfaces where cell potential is highest.
Generally, conventional deposition processes provide little control over thin film interface orientation and structure. Thin film interfaces formed by these methods are typically driven by natural nucleation and growth processes which produce undesirable film layer orientations, layer mismatches, and poor interfacial contact and bonding, resulting in lower effective contact area between layers. The resulting interfaces produced by conventional methods create impediments to rapid and efficient ion transport through the cell and result in increased cell impedance and reduced charge capacity. Additionally, due to impediments to ion transfer across the electrode-electrolyte interface, such interfaces are intrinsically unstable due to ion buildup at the interface which produces a high space charge density and higher electric field. These conditions may lead to decomposition reactions at the electrolyte-electrode interface with a resulting lower decomposition potential for the thin film cell and battery.
In addition to crystallographic orientation and interface structure, the crystallinity of as-deposited anode, cathode and electrolyte films is critical to overall performance in thin film batteries. Generally, with anisotropic materials, isotropic ion transport through fine grain, polycrystalline films with random grain orientation is faster than ion transport through coarse grain films which are strongly oriented. Since thin film battery structures are intrinsically anisotropic, in that they require ion transport in an orthogonal direction to the electrode-electrolyte interfaces, deposition processes which produce coarse grain films with c-axis orientations orthogonal to the electrode-electrolyte interfaces are undesirable. Conventional deposition methods which produce amorphous deposits may require subsequent, post-deposition annealing treatments to crystallize the as-deposited films. Generally, there is little control over post-anneal crystallographic orientations with these methods as the orientation of recrystallized, as-deposited films is typically established by natural growth processes leading to undesirably oriented films. Additionally, such treatments typically require high temperatures which can damage underlying film layers by promoting reactions between film layers or grain growth and coarsening of film layers. Deposition methods which produce deposits with coarse grain structures are more likely to form films with undesirable crystallographic orientations dominated by natural nucleation and grain growth processes.
In addition to orientation, interface structure and crystallinity requirements, the component film layers and layer interfaces in thin film cells and batteries must be both thermomechanically and “electromechanically” stable. Film layers and their interfaces must be sufficiently robust to withstand anticipated temperature changes encountered both during fabrication and operation of the batteries. For thermomechanical stability, thermal expansion coefficients and differences between thermal expansion coefficients of film layers and substrate materials must be factored in material selection to avoid thermal expansion mismatches which may produce sufficiently large stresses to cause cracking within the films or at film interfaces. For electromechanical stability, since solid state battery performance requires reversible transport, storage and removal of large quantities of ions during normal operation, solid-state phase transformations and accompanying volume changes must be considered in electrode material selection to minimize film expansion and associated strain which may produce sufficiently large stresses to cause cracking within film layers or between layers. In addition to material considerations, deposition methods which provide for dense, non porous films with minimal defects, strong interfacial bonding and intimate contact between film layers would be particularly advantageous for minimizing film or interfacial fractures caused by anticipated thermomechanical and electromechanical strain and stress.
Thus, a deposition method which provides for overcoming natural thin film nucleation and growth processes for control of crystallographic orientation, interface structure and bonding, crystallinity and grain sizes in thin film deposits would be particularly advantageous for the development and commercialization of thin film batteries.
A particularly useful review of current solid-state, thin film battery technology is disclosed in Julian, et al., Solid State Batteries: Materials Design and Optimization, Kluwer Academic Publishers (Boston, Mass., 1994) which is incorporated herein by this reference.
Commercial lithium batteries are well known in the art. Due to their relatively high energy density, voltage, and charge capacity, these bulk batteries are currently used as power sources for portable electronic devices, such as cameras, wireless phones and laptop computers, and computer motherboard CMOS EPROMs. Commercial lithium batteries typical employ liquid electrolytes which have a low reduction potential and are unstable over a range of voltage cycling, either decomposing or reacting with cell electrodes. In these batteries, porous polymer composite anodes are employed which are fabricated from blends of conductive graphite powder, lithium intercalatable graphite powder, polymer binders, and fugitive liquids which impart porosity to the anode. The porous anode is typically infiltrated with a corrosive liquid electrolyte which reacts with the carbon particulate and forms a solid residue, which is both an electronic and ionic insulator, on the surface of the carbon particles, resulting in an irreversible loss in reversible charge capacity. While the use of carbon particulate in these batteries provides for a very high electrolyte-anode surface area for lithium exchange, the high surface area accelerates reaction with the corrosive electrolyte. These batteries further employ porous polymer composite cathodes fabricated from blends of lithium intercalatable cobalt oxide powders, conductive carbon particulate, polymer binders, and fugitive liquids which impart porosity to the cathode. Since both electrodes require conductive carbon particle-to-particle contact and intimate electrolyte contact with intercalatable carbon and cobalt oxide powders, during repeated cycling, irreversible reaction of the electrolyte with both the anode and cathode particulates cause both passivation of the intercalatable powders and a reduction in particulate size of the conductive powders. This creates a dramatic change in internal cell resistance during repeated cycling, due to loss of both electronic and ionic conductivity, and an irreversible loss in the charge capacity. Thus, the lifetimes in these batteries are typically limited to between 500 to 1000 charge cycles.
More recently, development of thin film, solid-state lithium batteries is being pursued as replacements for current commercial lithium ion batteries for portable power sources in electronic devices and electric vehicles. Solid-state lithium batteries offer distinct advantages over conventional liquid or polymer electrolyte batteries due to the elimination of corrosive electrolyte. Due to the possibility for substantial improvement in reversible charge capacity and battery life as well as significant reduction is cell weight and volume, thin film solid-state lithium batteries are particularly promising for applications in electronic devices, electric vehicles and solid state device power supplies where space and weight are restricted and extended battery life, high energy density and high specific energy are required. A relative comparison of the energy densities and specific energies typically obtained with prior art commercial batteries is provided in FIG. 7. As shown in this figure, the energy densities (1000 Watt-hr/liter) and specific energies (500 Watt-hr/kg) anticipated with thin film lithium battery of the present invention offer substantial improvements over current commercial batteries.
In U.S. Pat. Nos. 5,338,625, 5,512,147, 5,569,520, 5,597,660 and 5,612,152 to Bates, et al., disclose a lithium thin film battery and an electrolyte material for lithium batteries. However, the lithium thin film battery disclosed by Bates, et al., has certain limitations due to the use of lithium metal anodes. U.S. Pat. No. 5,512,387 to Ovshinsky discusses several intrinsic technical and safety limitations of thin film batteries which incorporate lithium metal anodes, this patent being incorporated herein by this reference.
In U.S. Pat. No. 5,338,625, Bates, et al., disclose the use of a lithium phosphorus oxynitride electrolyte, LixPOyNz where x is approximately 2.8, 2y+3z is approximately 7.8 and z ranges between 0.16 and 0.46, which has been shown to be useful in lithium battery applications due to the relative high ionic conductivity and stability of the electrolyte over the range of lithium half cell voltages. However, the sputtering method employed by Bates, et al., for LiPON electrolyte deposition has fairly low deposition rates and requires long deposition times to obtain acceptable electrolyte film thickness and density. These deposition rates are generally impractical for commercial production of thin film batteries. Furthermore, the use of highly reactive lithium metal anodes by Bates, et al., compromises selection of compatible component materials and restricts the choice of processing methods.